1. Field of the Invention
This invention relates, in general, to a radio-communication apparatus. In particular, the invention relates to an apparatus that carries out radio-communication with data carriers, e.g., RFID (Radio Frequency Identification) tags, and reads and/or writes data from and/or into such data carriers. The invention also relates to a method for enabling radio-communication between a radio-communication apparatus and data carriers.
2. Description of the Related Art
In recent years, RFID labels draw one's attention to manage articles to be sold, for example. RFID label includes a label sheet and an RFID tag well known in the art, having an IC chip and an antenna and the RFID tag is attached to the label sheet. RFID label has advantages that a large number of data are stored in the IC chip, data is read and/or written from and/or to the IC chip with non-contact manner using an electromagnetic waves and thus influence of stain and/or dust can be prevented, and data stored in each IC chip of a plurality of RFID labels can be read collectively within a short time.
Such RFID labels are attached to respective articles to be sold and information regarding each article is stored in IC chip of respective RFID labels. In this case, it is required to print information such as an article name on the surface of each label to enable a visible inspection or recognition by an operator or a consumer.
Japanese laid-open (kokai) patent No. 2005-332318 discloses a printing apparatus that uses a continuous rolled base sheet on which a plurality of RFID labels are peelably arranged in line. Using a radio-communication, data is stored in an IC chip of respective RFID labels which are sequentially conveyed along with the base sheet while the base sheet is wound up and, a required information is printed on respective RFID labels m sequence.
A usable frequency band of electromagnetic waves of UHF band in RFID technology is different in every country or region. For example, a usable frequency band is 950 MHz to 956 MHz in Japan, 902 MHz to 928 MHz in U.S.A. and 862 MHz to 868 MHz in Europe, respectively
Thus, a radio-communication apparatus commonly usable in such countries or regions needs a large external size of an antenna that covers all of the above-described frequency bands or to selectively use antennas each of which is designed exclusively for each country or region. It is difficult to employ however, such a large antenna to an apparatus whose external size is required to be small in the former case and a cost is increased to prepare such plural kinds of antenna in the latter case.
The above-described Japanese laid-open (kokai) patent also discloses a radio-communication apparatus that a dielectric element is located at a position through which electromagnetic waves radiated from the antenna pass and a distance from the dielectric element to the antenna is controlled depending on the country or region where the apparatus is used to adjust the frequency characteristic of the antenna to a specific frequency band of the country or region. This operation is based on a phenomenon that the frequency characteristic of an antenna varies with influence of a dielectric element against the antenna when the dielectric element is located near the antenna. Therefore, the above-described problems can be solved by the Japanese laid open (kokai) patent.
In the meantime, as described above, a usable frequency band of electromagnetic waves of UHF band in RFID technology is respectively determined in each country or region. Thus, it is required to design RFID tags so that, in RFID labels using the RFID tags, read and/or write of data can be performed precisely with electromagnetic waves of a specific frequency band that is determined in the country or region where RFID labels are used.
In general, RFID labels are attached to various articles and thus it is well known that a specific frequency band, of RFID tags may greatly vary with a dielectric factor of articles to which RFID labels are respectively attached.
For example, in a case that RFID label is attached to an article that is made of a relatively thick paper material e.g., hook, the optimum frequency of the RFID tag decreases about 50 MHz from an original optimum frequency. In Japan, since a usable frequency band, of electromagnetic waves of UHF band is determined 950 MHz to 956 MHz, RFID tag is exclusively designed such that the optimum frequency thereof is to be 1,0000 MHz, considering beforehand the fact that the optimum frequency decreases about 50 MHz therefrom (1,000 MHz) as described above when the RFID label on which the RFID tag is mounted is attached to a book.
Similarly in a case that RFID label is attached to an article that is made of a relatively thick plastic material, e.g., plastic box, the optimum frequency of RFID tag decreases about 70 MHz from an original optimum frequency thereof. Therefore, RFID tag is exclusively designed such that the optimum frequency is determined to 1,020 MHz, considering that the optimum frequency practically decreases about 70 MHz therefrom (1,020 MHz) when the RFID label on which the RFID tag is mounted is attached to a plastic box.
Furthermore, in a case that RFID label is attached to an article made of a thin material, e.g., article tag, or is used, alone, the optimum frequency of such RFID tags is determined to 950 MHz. This is because that the optimum frequency thereof does not vary even if the RFID label is attached to such a thin material.
As described above, optimum frequencies of RFID tags are different from one another depending on. RFID labels exclusively manufactured for specific articles to which RFID labels are attached.
Therefore, the optimum frequency of RFID tags varies depending on articles to which RFID labels are attached even in the same country or region. In response to this circumstance, it is understood that a dielectric material is located near the antenna and a distance between the antenna and the dielectric material is controlled at a precise value to adjust the optimum frequency of the antenna to a usable frequency band in the country or region where RFID labels are used.
However, it is troublesome operation to manually regulate the distance between the antenna and the dielectric material to adjust the optimum frequency of RFID tag every time a kind of RFID label is changed. A practical solution is desired.